I. Field
The present disclosure relates generally to amplifiers, and more specifically to gain control in low-noise amplifiers.
II. Background
A wireless device (e.g., a cellular phone or a smartphone) in a wireless communication system may transmit and receive data for two-way communication. For example, the wireless device may operate in a frequency division duplexing (FDD) system or in a time division duplexing system (TDD). The wireless device may include a transmitter for data transmission and a receiver for data reception. Thus, the wireless device may process both analog and digital signals in order to provide communication and/or data services.
Receivers in wireless devices are used to receive and demodulate received RF signals. A typical receiver includes a low noise amplifier (LNA) to amplify received RF signals prior to demodulation. Impedance matching, isolation, and linearity are the major concerns for LNAs in receivers. For example, it is desirable that the LNA provide an input impedance, typically 50 ohms, to match the impedance of a receiving antenna. The LNA should also provide accurate gain control so that the amplification applied to the received RF signals prior to demodulation can be accurately controlled.
Unfortunately, conventional LNAs may utilize several external components in order to provide the desired input impedance. This increases the circuit area required by the LNA. Also, accurate gain control may be a problem since adjustments to gain in a convention LNA may affect the input impedance.
It is therefore desirable to have a low power amplifier having gain control with constant input impedance that overcomes the disadvantages of conventional circuits.